Overall effectiveness is a measure of the actual metal temperature of the wall and through careful scaling it is possible to conduct rig scale experiments to infer representative metal temperatures at engine cycle conditions. A number of the major heat transfer mechanisms present in the engine are included in these experiments, including the cooling film providing the hot side convective heat transfer driving temperature, conduction through the metal wall together with the internal effusion geometry providing a convective heat transfer path out of the wall and into the cooling flow passing through the passages. The radiative loads associated with the combustion flame are not accounted for in these experiments, nor is the convective heat transfer caused by cold side cross flow within the feed annuli surrounding the combustor. In order to understand the influence a number of parameters have on the overall effectiveness and hence the metal temperature, experiments directed at the effect of freestream turbulence levels, momentum ratio and cooling passage geometry are conducted on the 6 test plates previously discussed. As the adiabatic film effectiveness results show little discernible difference is caused by the blockage characteristics of a mixing port, this test condition is not repeated for the overall effectiveness measurements. Results are again presented as both 2D surface contours of the overall effectiveness and spanwise averaged over the same central strip as illustrated earlier in Figure 62. Surface contours for the highest and lowest momentum ratio conditions at the two freestream turbulence conditions are presented in every other numbered figure from Figure 89 to Figure 100 with spanwise averaged plots presented in the remaining figures in this number range. The only exception to this is the Rectilinear Helical geometry which was only tested at the 20% freestream turbulence intensity condition. Surface contours for all tested conditions are also presented in Appendix A.2.
5.2.1 Freestream Turbulence
Spanwise averaged plots of overall effectiveness are presented below with solid lines representing the lower 5% turbulence case and the dashed lines representing the higher 20% turbulence intensity condition. Unlike with the adiabatic film effectiveness results, freestream turbulence intensity has a notable effect on overall effectiveness with all tested geometries showing a decrease in overall effectiveness at the higher turbulence condition. However, from the adiabatic film effectiveness study it can be seen that the driving temperature is not
effectiveness and hence reduced gas temperature at high turbulence levels and low blowing ratios. Therefore the increase in metal temperature must be due to the increased heat transfer coefficient on the plate surface caused by the increased turbulent flow intensity. This indicates that while no difference was seen in the adiabatic effectiveness results when the port blockage simulant was used, the associated increase in freestream turbulence intensity level might have a strong influence on overall effectiveness. This also shows that adiabatic effectiveness alone is not enough to fully characterise the cooling of the wall under a convective heat load, with further information of HTC distribution required to give better understanding of actual wall temperatures.
5.2.1.1 Cylindrical Effusion Geometry
From the 2D surface contours of overall effectiveness presented in Figure 89 it can be seen that through increasing the freestream turbulence level the maximum effectiveness decreases. From interrogation of the spanwise averaged results, there is a maximum decrease in the overall effectiveness of around 0.03 and which is consistent for all coolant flow rates. The data from the two turbulence conditions remains roughly equal up until around the second row of cooling holes, at which point the data begin to diverge, with the effectiveness of the low turbulence conditions continuing to rise steadily along the plate length before reaching a maximum around the seventh row. Performance then falls as the end of the array is reached and no more coolant holes are present to remove heat from within the wall. A similar trend is found at low momentum ratios and high turbulence. However as the flow rate increases at high turbulence levels, a double peak is seen in the spanwise averaged data coinciding with a second patch of higher effectiveness between the first and second rows; this is believed to be caused by a local increase in HTC after the second row. As the film performance of the cylindrical hole is relatively poor, the local temperature in this region is still high but through introducing the coolant into the mainstream, the flow downstream of the injection site is more unsteady causing an increase in local HTC as seen by Martin. (57) The combination of increased local HTC with poor coolant coverage results in an area of increased heat transfer into the plate and hence higher local surface temperatures. As the coolant film builds up, the increase in local HTC is offset by the cooler driving temperature and the heat transfer is decreased again, reducing the plate temperature downstream of the 4th row.
5.2.1.2 Spey Fan Effusion Geometry
At the lowest momentum ratio the overall effectiveness of the Spey fan geometry is relatively unaffected by the freestream turbulence intensity, with both distributions of spanwise averaged overall effectiveness almost collapsing. However the adiabatic film effectiveness plots at the same conditions show a difference in driving temperature, particularly towards the end of the plate. From this it may be surmised that the contribution to heat transfer caused by turbulent mixing of the coolant and its reduction in the driving temperature is roughly equal to the increase in HTC caused by the same process. At higher momentum ratios the adiabatic film effectiveness is not affected by the increased turbulence levels, therefore the increased HTC causes the heat transferred into the wall to be increased and hence reduces overall effectiveness by up to 0.02.
5.2.1.3 Modified Fan Effusion Geometry
As the adiabatic film performance of the Modified fan shows little sensitivity to freestream turbulence intensity at all of the coolant flow conditions here, the overall effectiveness results indicate that once again the increased HTC on the surface caused by the local turbulent conditions has the effect of reducing the overall effectiveness and increasing the wall temperature. At the lowest momentum ratio the difference between low and high turbulence cases is around 0.02, rising to 0.03 at all other cooling flow conditions.
5.2.1.4 Slotted Effusion Geometry
As for the cases above, the effect of increasing the freestream turbulence intensity is to reduce the overall effectiveness. The Slotted design showed reduced adiabatic film effectiveness at increased turbulence levels, resulting in a difference in overall effectiveness of 0.031 at the lowest coolant flow condition reducing to 0.026 at the highest momentum ratio. This reduction in difference is a result of the reduced effect of freestream turbulence on the adiabatic film performance at higher coolant momentum ratios. A double peak similar to that found in the cylindrical case data is also seen in the spanwise averaged plot, indicating a similar increase in local HTC downstream of the first row of coolant holes in an area where cooling film performance is poor. This is supported by the 2D surface contours of the high turbulence, high flow condition where clear streamwise streaks are visible downstream of each of the cooling holes. Here film performance is locally good enough to compensate for the increased HTC caused by the introduction of coolant onto the surface. However in the area between coolant
holes where the film is poor, the localised temperature is higher and so the spanwise average is lower than the area upstream of the first row. In these regions the internal heat removal is high enough to reduce the temperature of the plate before the coolant has been introduced and the HTC augmented.
5.2.1.5 Circular Helix Effusion Geometry
The Circular Helical design also follows the same trend of reduced overall effectiveness at increased freestream turbulence levels. The only exception being at the lowest momentum ratio where the increased turbulence case shows slightly higher spanwise averaged overall effectiveness up to the seventh row of coolant holes. The adiabatic film effectiveness shows a similar trend at this same blowing condition, with the spanwise average increasing from the 5% to the 20% turbulence intensity condition. Therefore the difference in overall effectiveness is attributed to the difference in gas temperature at the wall driving the convective heat transfer. The performance difference between high and low turbulence conditions increases with increasing momentum ratio up to a maximum of 0.015.
5.2.2 Effect of Coolant Momentum Ratio
In addition to improvements in the film performance discussed in the previous section, increasing the wall momentum ratio and hence coolant flow rate through the cooling passages removes more heat from the wall internally. Since conventional effusion cooling designs rely on simple straight through holes, little of the cooling potential of the coolant air is utilised. The DLD process allows the internal passages to be designed in such a way as to increase the amount of heat removed by the coolant through more complex internal flow passages before being ejected onto the surface to form the protective film layer. This section identifies how altering the wall momentum ratio influences the overall effectiveness as measured on the hot- side surface. Referring to the adiabatic effectiveness plots discussed earlier, it can be seen that for the majority of designs tested a maximum film performance is reached at a momentum ratio of ≈ 6, equivalent to a liner pressure drop of 1.0% ∆𝑃𝑃 𝑃𝑃⁄ at the engine cycle conditions considered here. Therefore, any difference in overall effectiveness at the higher flow rates must be attributable to the internal heat removal mechanism. Overall effectiveness data will collapse onto a single line as the maximum cooling potential of a given solution is reached. Spanwise averaged results are shown in odd numbered figures from Figure 90 to 100 alongside the 2D surface contours at the highest and lowest momentum ratio conditions. All surface
contours are also presented in Appendix A.2. Values quoted in the following sections relate to the 20% freestream turbulence intensity case only.
5.2.2.1 Cylindrical Effusion Geometry
Adiabatic film effectiveness shows little sensitivity to momentum ratio at the conditions tested. However, there is a notable increase in overall effectiveness as momentum ratio increases. This increase is therefore a result of increased HTC over the surface of the cooling holes within the wall. By increasing the effusion wall blowing ratio, the velocity of the flow through the passages is increased which in turn increases the transfer of heat from the wall and into the coolant through forced convection. From Figure 90 it can be seen that the greatest increase occurs between the lowest two blowing conditions, with the difference in peak spanwise averaged effectiveness reducing from around 0.02 for the lowest to about 0.003 (within the margin of uncertainty) for the highest momentum ratio condition. This suggests a state of diminishing returns has been reached with further increases in coolant flow having little benefit. This optimum condition is the point at which relatively good film coverage is achieved. However, upstream of the first row of cooling holes there is a large difference between all coolant flow conditions. Since this is at the upstream edge of the array before any coolant has been introduced onto the surface, the major mechanism for heat removal is via the internal cooling passages and therefore the large difference between overall effectiveness indicates the performance benefits achievable through good internal cooling design.
5.2.2.2 Spey Fan Effusion Geometry
As indicated earlier, the adiabatic film effectiveness is largely insensitive to momentum ratio at the higher freestream turbulence levels and therefore the difference seen in overall effectiveness is caused by the removal of heat through the internal flow passages. The benefit of increasing blowing ratio is again subject to diminishing returns. The difference between peak spanwise averaged overall effectiveness values decreases from 0.018 for the lowest blowing ratio conditions to 0.002 (within the margin of uncertainty) for the highest. This difference is dependent on the adiabatic film effectiveness, with a larger difference notable in areas with relatively poor film coverage. In this particular case, upstream of the third row of cooling holes, where more heat is transferred into the wall and therefore can be removed by the internal flow passages.
5.2.2.3 Modified Fan Effusion Geometry
As for the Spey fan case, the same pattern is seen for the Modified fan geometry. With the relatively small sensitivity of film performance to momentum ratio, yet notable sensitivity of overall effectiveness, it may be concluded that the internal cooling performance is improved with increased momentum ratio. Again this improvement in performance is subject to diminishing returns as the wall blowing ratio increases. The difference between peak spanwise averaged overall effectiveness at the lowest conditions is 0.03 and between the two highest conditions this improvement drops to 0.004 (within the margin of uncertainty). Due to the increased lay angle from 17° to 21° of the holes in this design, the internal passage length is shorter than any of the other geometries tested. As a result, the performance differences between blowing conditions are larger as the internal cooling mechanism is less efficient and therefore more sensitive to coolant flow rate.
5.2.2.4 Slotted Effusion Geometry
As seen in Figure 80, there is a noticeable difference in adiabatic film performance with momentum ratio for the Slotted design, particularly at the lower flow rates tested. However, due to the long passage lengths and wider Slotted cross section, the overall effectiveness difference between conditions is not as large as for the traditional cylindrical cross section geometries discussed previously. The differences in peak overall effectiveness are 0.016 and 0.002 (within the margin of uncertainty) between the two lowest and highest momentum ratio conditions respectively. This indicates that a good internal cooling geometry can compensate in part for a poorer cooling film coverage.
5.2.2.5 Circular Helix Effusion Geometry
The Circular Helical geometry is the first geometry to be designed to maximise the heat transfer through the internal flow passage. As a result, the overall effectiveness is high even at lower cooling flow rates, with a difference between the two lowest peak spanwise averaged values of 0.011 reducing to less than 0.001 (within the margin of uncertainty) for the two highest momentum ratio conditions. This very small difference suggests that the maximum potential of this geometry is reached at much lower momentum ratios than for the geometries discussed above. For example, the overall effectiveness data indicate the difference between the highest and second lowest conditions is less than 0.002, which is less than the margin of uncertainty. The performance difference up to the third row of cooling holes is much smaller than is
observed for those geometries discussed above, with no discernible difference between results at the two highest blowing rates.
5.2.2.6 Rectilinear Helix Effusion Geometry
The Rectilinear Helical geometry represents an attempt to further improve the contribution of internal heat transfer to the overall effectiveness performance through the use of a square cross section promoting stronger secondary flows. Examining the spanwise averaged plot of overall effectiveness it can be seen that this geometry shows similar performance to that of the Circular Helical design at all conditions. At the two lowest momentum ratio conditions, the difference between peak spanwise averaged overall effectiveness is 0.009, reducing to less than 0.002 (within the margin of uncertainty) between the two highest. As the adiabatic film effectiveness is more or less equal at all flow conditions, particularly the three higher blowing ratios, the data shows that this cooling system reaches its maximum potential at much lower flow rates than the more conventional straight through passages.
Low BR ~0.5%𝛥𝛥𝑃𝑃/𝑃𝑃 𝑇𝑇𝑇𝑇 High BR ~2.5%𝛥𝛥𝑃𝑃/𝑃𝑃 5% 𝑴𝑴𝑫𝑫 = 𝟑𝟑. 𝟔𝟔𝟎𝟎 𝑴𝑴𝑫𝑫 = 𝟏𝟏𝟐𝟐. 𝟔𝟔𝟏𝟏 20% 𝑴𝑴𝑫𝑫 = 𝟑𝟑. 𝟐𝟐𝟏𝟏 𝑴𝑴𝑫𝑫 = 𝟏𝟏𝟔𝟔. 𝟎𝟎𝟑𝟑 𝜼𝜼𝑵𝑵𝒐𝒐
Figure 89 - Cylindrical hole overall effectiveness at various turbulence and blowing ratio conditions
Low BR ~0.5%𝛥𝛥𝑃𝑃/𝑃𝑃 𝑇𝑇𝑇𝑇 High BR ~2.5%𝛥𝛥𝑃𝑃/𝑃𝑃 5% 𝑴𝑴𝑫𝑫 = 𝟑𝟑. 𝟗𝟗𝟎𝟎 𝑴𝑴𝑫𝑫 = 𝟏𝟏𝟐𝟐. 𝟗𝟗𝟏𝟏 20% 𝑴𝑴𝑫𝑫 = 𝟑𝟑. 𝟐𝟐𝟔𝟔 𝑴𝑴𝑫𝑫 = 𝟏𝟏𝟏𝟏. 𝟐𝟐𝟏𝟏 𝜼𝜼𝑵𝑵𝒐𝒐
Figure 91 – Spey fan overall effectiveness at various turbulence and blowing ratio conditions
Low BR ~0.5%𝛥𝛥𝑃𝑃/𝑃𝑃 𝑇𝑇𝑇𝑇 High BR ~2.5%𝛥𝛥𝑃𝑃/𝑃𝑃 5% 𝑴𝑴𝑫𝑫 = 𝟑𝟑. 𝟔𝟔𝟑𝟑 𝑴𝑴𝑫𝑫 = 𝟏𝟏𝟐𝟐. 𝟑𝟑𝟏𝟏 20% 𝑴𝑴𝑫𝑫 = 𝟑𝟑. 𝟏𝟏𝟗𝟗 𝑴𝑴𝑫𝑫 = 𝟏𝟏𝟏𝟏. 𝟑𝟑𝟎𝟎 𝜼𝜼𝑵𝑵𝒐𝒐
Figure 93 – Modified fan overall effectiveness at various turbulence and blowing ratio conditions
Low BR ~0.5%𝛥𝛥𝑃𝑃/𝑃𝑃 𝑇𝑇𝑇𝑇 High BR ~2.5%𝛥𝛥𝑃𝑃/𝑃𝑃 5% 𝑴𝑴𝑫𝑫 = 𝟑𝟑. 𝟐𝟐𝟏𝟏 𝑴𝑴𝑫𝑫 = 𝟏𝟏𝟐𝟐. 𝟐𝟐𝟏𝟏 20% 𝑴𝑴𝑫𝑫 = 𝟑𝟑. 𝟏𝟏𝟔𝟔 𝑴𝑴𝑫𝑫 = 𝟏𝟏𝟏𝟏. 𝟎𝟎𝟏𝟏 𝜼𝜼𝑵𝑵𝒐𝒐
Figure 95 – Slotted overall effectiveness at various turbulence and blowing ratio conditions
Low BR ~0.5%𝛥𝛥𝑃𝑃/𝑃𝑃 𝑇𝑇𝑇𝑇 High BR ~2.5%𝛥𝛥𝑃𝑃/𝑃𝑃 5% 𝑴𝑴𝑫𝑫 = 𝟑𝟑. 𝟔𝟔𝟐𝟐 𝑴𝑴𝑫𝑫 = 𝟏𝟏𝟏𝟏. 𝟎𝟎𝟏𝟏 20% 𝑴𝑴𝑫𝑫 = 𝟑𝟑. 𝟐𝟐𝟎𝟎 𝑴𝑴𝑫𝑫 = 𝟏𝟏𝟏𝟏. 𝟗𝟗𝟏𝟏 𝜼𝜼𝑵𝑵𝒐𝒐
Figure 97 – Circular helix overall effectiveness at various turbulence and blowing ratio conditions
Low BR ~0.5%𝛥𝛥𝑃𝑃/𝑃𝑃 𝑇𝑇𝑇𝑇 High BR ~2.5%𝛥𝛥𝑃𝑃/𝑃𝑃
20%
𝑴𝑴𝑫𝑫 = 𝟑𝟑. 𝟑𝟑𝟎𝟎 𝑴𝑴𝑫𝑫 = 𝟏𝟏𝟏𝟏. 𝟑𝟑𝟏𝟏
𝜼𝜼𝑵𝑵𝒐𝒐
Figure 99 – Rectilinear helix overall effectiveness at various turbulence and blowing ratio conditions
5.2.3 Effusion Hole Geometry
In this section the overall effectiveness performance measured for each effusion hole geometry are compared. As previously mentioned, the first row of cooling holes are not located in the same streamwise position along the plate in all designs, with the Cylindrical holes significantly closer to the leading edge of the plate than for the other cases. As these are conjugate heat transfer experiments, this difference in geometry is more significant than for the adiabatic film effectiveness tests because the boundary between the plate and the mounting frame is much closer to the internal cooling passages in the cylindrical design. This results in the shallower initial gradient in spanwise effectiveness with streamwise position and therefore it is difficult to draw conclusions about the relative performance of this geometry to the others in this upstream area. However, this boundary condition should have little to no effect on the results obtained mid-way along the plate, allowing comparisons to be made in this region. All other plates have the cooling holes located in roughly the same initial position and can be compared over the entire data set more easily. As with the adiabatic film effectiveness study, results are compared at the 20% freestream turbulence intensity conditions and at momentum ratios of around 3 and 15 equivalent to ∆𝑃𝑃 𝑃𝑃⁄ = 0.5% and 2.5% at engine cycle conditions respectively. Figure 101 presents 2D surface contour data at the highest momentum ratio condition with spanwise averaged data plotted in Figure 102. From the spanwise averaged overall effectiveness data it can be seen that the geometries fall into three rough groupings. First, the cylindrical geometry acts as the baseline, with the lowest performance over the length of the plate and spanwise averaged effectiveness in the 0.80-0.85 band. Next, the geometries which promote good film performance improve on the base line with spanwise averaged performance levels in the region of 0.85-0.90. Finally, the helical geometries which combine good film performance with strong internal heat removal return the best performance with spanwise averaged values of 0.90-0.95. Due to the different array pattern used by the Slotted geometry it does not fit neatly into any of the groups but instead bridges the gap between the film only and internally improved designs. This difference in overall effectiveness infers a reduction in minimum metal temperature of around 100K when scaled back up to engine cycle conditions. By packing a large number of cooling passages in the same spanwise location, a significant